1. Field of the Invention
The present invention relates to a progressive multifocal lens used to assist an accommodation power of an eye.
2. Related Background Art
As spectacle lenses for assisting an accommodation power when an accommodation power of an eye becomes too weak to see a near point, various progressive multifocal lenses have been known. A progressive multifocal lens of this type has a "farsight portion", a "nearsight portion" and a progressive region (referred to as an "intermediate portion" hereinafter) which is located between the above two portions in which a refracting power is continuously changed. When the progressive multifocal lens is actually worn as spectacles by a user, the farsight portion becomes an upper portion while the nearsight portion becomes a lower portion. In the present invention, it is to be noted that "upper portion" "lower portion", "horizontal" and "vertical" are used to point positions of the progressive multifocal lens in the state that it is actually worn by the user. For example, the lower portion of the farsight portion is a position within the farsight portion close to the intermediate portion. Also, the difference between the nearsight refracting power and the farsight refracting power is called an additional power.
In a progressive multifocal lens of this type, in general, when wide distinct vision areas (ranges with an astigmatic difference of 0.5 diopter or less) of the farsight and nearsight portions are kept and coupled by the progressive power range, lens aberrations are concentrated on side regions of the progressive power range. As a result, blurring and distortion of an image occur in these side portions. Therefore, when a user wears spectacles formed of these multifocal lenses and shifts his eyes laterally, he perceives the distortion of an image as the fluctuation of the image and has an unpleasant feeling.
In order to prevent such a problem of visual characteristics, in known progressive multifocal lenses, various designs and evaluations have been made from a variety of viewpoints. As for the shape of a lens surface, an intersecting line (principal meridional curve) defined by a section extending along a vertical meridian vertically passing through substantially the center of a lens surface and an object-side lens surface is used as a reference curve for expressing specifications such as an additional power of a lens, and is also used as an important reference curve in a lens design.
As shown in FIG. 1, a progressive multifocal lens (hereinafter referred to as an asymmetrical progressive multifocal lens) has been proposed in which an intermediate portion P and a nearsight portion N are asymmetrically arranged in consideration of the fact that a nearsight eye point (not shown) of the nearsight portion N is slightly displaced from the center toward a nasal side when the lens is worn. Also, in such an asymmetrical progressive multifocal lens, a center line MM' consisting of an intersecting line of an object-side lens surface and a section passing the center OF of a farsight portion F, a farsight eye point E and the center of the nearsight portion N is used as a reference line. These reference lines will be referred to as "principal meridional curves" in the present invention.
There are conventional progressive multifocal lenses disclosed in, e.g., Japanese Patent Publication Nos. 63-42764 and 2-39768.
In the lens of Japanese Patent Publication No. 63-42764, the distortion of an image in side regions of an intermediate portion is small and the distribution of an astigmatic difference becomes gentle. However, the reduction of a maximum astigmatic difference is not achieved.
In the lens of Japanese Patent Publication No. 2-39768, the width of a distinct vision area is enlarged by lessening the rates of changes of curvatures of horizontal crossing curves in farsight and nearsight portions toward the upper and lower regions of the lens. However, as the result of widening the distinct vision area, the gradient of an astigmatic difference becomes steep especially in the nearsight portion and the reduction of a maximum astigmatic difference is not achieved.
Thus, although the improvement of visual performance can be admitted to a certain extent in the conventional progressive multifocal lenses, it is not sufficient to obtain optimum visual performance in that the reduction of the maximum astigmatic difference is not yet achieved.